I draw your attention to subscription and unsubscribe.
I would call both of these the very definition of a meta tag. "Subscribing" could be to a mailing list (or similar), paying for a service, or just "watching" for some sort of serially released material. "Unsubscribe" is just the opposite action.
Further, there is no tag wiki or usage guidance for either.
Currently 18 questions for subscription.
3 about Google Groups 1 about Office 365 3 about YouTube 1 about Google Play Music 1 about Twitter 1 about Trello 1 about Posterous 1 about Facebook 1 about GitHub 1 about Feedburner 1 about SourceForge 1 about Blogger 1 about Google Calendar 1 about links in spam email
Currently 7 questions for unsubscribe.
1 about Backblaze 1 asking about a "reputation database" for "unsubscribe links" in email messages 1 about email messages from Facebook 1 about notifications from Facebook 2 about Trello 1 about a site called "Find Me Spot"
Neither tag could possibly stand on its own. Further, no question gains any extra clarity beyond the app tag(s) that are already on the question.
These tags should be "burninated".